A selective epitaxial deposition process is one in which monocrystalline material is deposited on predetermined portions of a substrate surface. In the selective epitaxial deposition of silicon, these predetermined portions may conveniently be defined by apertures in a mask which is disposed on the surface. For example, in a conventional process, a layer of masking material such as silicon dioxide is formed on a surface of a monocrystalline silicon substrate, apertures are formed in the mask, and monocrystalline silicon is deposited within the apertures. Further examples of selective epitaxial silicon deposition can be found in U.S. Pat. No. 3,425,879, METHOD OF MAKING SHAPED EPITAXIAL DEPOSITS, D. W. Shaw et al., Feb. 4, 1969; and in SELECTIVE EPITAXIAL DEPOSITION OF SILICON, B. D. Joyce et al., Nature, Aug. 4, 1962, pp. 485-486.
In commonly assigned U.S. patent application Ser. No. 608,544, METHOD FOR GROWING MONOCRYSTALLINE SILICON THROUGH A MASK LAYER, J. F. Corboy Jr. et al., filed May 10, 1984, now U.S. Pat. No. 4,578,142 a two stage selective epitaxial deposition process is described. Basically, this process comprises providing a substrate having an apertured mask thereon and subjecting the substrate to a two stage deposition cycle. In the first stage, silicon is deposited from a silicon-source gas. In the second stage, performed in situ, a portion of the deposited silicon is etched by subjecting the substrate to a silicon-etching gas. Using this process it is possible to form monocrystalline silicon islands within the mask apertures without forming silicon on the exposed surface of the mask layer. The patent application further indicates that a silicon-etching gas such as HCl may be included during the deposition stage of the cycle. It is disclosed that the silicon-etching gas present during the deposition stage influences the vertical growth rate, the horizontal-to-vertical growth rate ratio, and the likelihood of non-monocrystalline silicon forming on the mask during the deposition stage. However, a problem has been observed when using selective epitaxial deposition processes such as the above two stage process to form monocrystalline silicon deposits within apertures having significantly different geometries. In conventional processes the monocrystalline islands formed in the relatively small apertures are of different thickness from the islands formed in the relatively large apertures.